15th & 16th June 2021 A very warm welcome to the Permutation Patterns 2021 Virtual Workshop, hosted by the Combinatorics Group at the University of Strathclyde. Programme All workshop activities will take place online. Registered participants will receive a Zoom link for the sessions, along with links to the pre-recorded keynote addresses so they can be watched asynchronously. There will be four sessions on both days. During Sessions A, B and D (one hour), six speakers will each give a five minute presentation and then take questions. During Session C (thirty minutes), one of the keynote speakers will give a 10–15 minute summary of their address, followed by an opportunity for discussion. The full pre-recorded keynote address will be streamed online during the break immediately prior to Session C for the benefit of those who weren’t able to watch it earlier. Schedule The synchronous sessions will take place during the late afternoon and evening in Europe. The full schedule is on the following two pages, with links to all the abstracts. Session times use British Summer Time (UTC+1). Here are the times in some other locations. Session A Session B Session C Session D US Pacific 08.00 09.30 11.45 12.30 US Central 10.00 11.30 13.45 14.30 US Eastern 11.00 12.30 14.45 15.30 UK & Ireland 16.00 17.30 19.45 20.30 Central Europe 17.00 18.30 20.45 21.30 Israel 18.00 19.30 21.45 22.30 India 20.30 22.00 00.15 01.00 China 23.00 00.30 02.45 03.30 Eastern Australia 01.00 02.30 04.45 05.30 New Zealand 03.00 04.30 06.45 07.30 Proceedings A special issue of Enumerative Combinatorics and Applications (ECA) will be published to mark the conference. Full papers on any topic related to permutation patterns, broadly interpreted (and not restricted to results presented at the conference), are welcome to be submitted for consideration. Papers will be refereed in accordance with the usual standards expected of ECA. Submissions should be emailed to [email protected] by 1st December 2021. 1 Tuesday 15th June 2021 UK time (UTC+1) Welcome 16.00{16.05 Session A: Algorithms and computational complexity 16.05{17.05 Chair: Torsten M¨utze An automatic direct enumeration of Av(1342) | Emile´ Nadeau (p.89) Automated bijections with Combinatorial Exploration | Jon Steinn Eliasson (p.27) Pattern-avoiding rectangulations and permutations | Arturo Merino (p.87) Counting small patterns and testing for independence | Chaim Even-Zohar (p.34) Hardness of -permutation pattern matching | Michal Opler (p.90) C Sorting time of permutation classes | V´ıtJel´ınek (p.56) Session B: Classical avoidance and pattern densities 17.30{18.30 Chair: Jeff Liese Permutations with exactly one copy of a monotone pattern of length k, and a generalization | Alex Burstein (p.17) Permutations avoiding sets of patterns with long monotone subsequences | Mikl´osB´ona (p.13) Universal 321-avoiding permutations | Bogdan Alecu (p.10) Layered permutations and their density maximisers | Adam Kabela (p.63) Feasible regions and permutation patterns | Raul Penaguiao (p.93) Permutation limits at infinitely many scales | David Bevan (p.11) Conference photo Break 18.45{19.45 Lucas Gerin's pre-recorded keynote address will be streamed during the break. Session C: Keynote address 19.45{20.15 Chair: Lara Pudwell Patterns in substitution-closed permutations: a probabilistic approach | Lucas Gerin (p.45) Session D: Probability 20.30{21.30 Chair: Tony Mendez Increasing subsequences in random separable permutations | Valentin Feray (p.40) Fixed points of permutations avoiding increasing patterns | Erik Slivken (p.100) A simple proof of a CLT for vincular permutation patterns for conjugation invariant permutations | Mohamed Slim Kammoun (p.69) Two equators of the permutohedron | Joshua Cooper (p.23) A probabilistic approach to generating trees | Jacopo Borga (p.14) The density of Costas arrays decays exponentially | Lutz Warnke 2 Wednesday 16th June 2021 UK time (UTC+1) Session A: Algebra, permutations and words 16.00{17.00 Chair: Alex Burstein Rowmotion on 321-avoiding permutations | Ben Adenbaum (p.4) Permutation groups and permutation patterns | Erkko Lehtonen (p.75) Spherical Schubert varieties and pattern avoidance | Christian Gaetz (p.44) Some combinatorial results on smooth permutations | Shoni Gilboa (p.48) On the existence of bicrucial permutations | Tom Johnston (p.62) Qubonacci words | Sergey Kirgizov (p.74) Session B: Permutation statistics 17.30{18.30 Chair: Bruce Sagan Pattern avoidance in cyclic permutations | Jinting Liang (p.80) The bivariate generating function on the statistics Peak and Des for cyclic permutations on [n + 2] which avoid the patterns [1324] and [1423] | James Schmidt (p.98) Admissible pinnacle sets and ballot numbers | Rachel Domagalski (p.24) A formula for counting the number of permutations with a fixed pinnacle set | Quinn Minnich (p.88) A new algorithm for counting the admissible orderings of a pinnacle set | Alexander Sietsema (p.99) New refinements of a classical formula in consecutive pattern avoidance | Yan Zhuang (p.110) Break 18.45{19.45 Luca Ferrari's pre-recorded keynote address will be streamed during the break. Session C: Keynote address 19.45{20.15 Chair: Rebecca Smith Sorting with stacks and queues: some recent developments | Luca Ferrari (p.42) Session D: Permutations and patterns 20.30{21.30 Chair: Vince Vatter Sorting with a popqueue | Lapo Cioni (p.18) Triangular permutation matrices | Vadim Lozin (p.81) On SIF permutations avoiding a pattern | Michael D. Weiner (p.108) On pattern avoidance in matchings and involutions | Justin M. Troyka (p.101) Bijections for derangements and pattern-avoiding inversion sequences | Sergi Elizalde (p.29) Classical pattern-avoiding permutations of length 5 | Anthony Guttmann (p.54) Closing 21.30{21.35 3 Permutation Patterns 2021 Virtual Workshop Wednesday Session A Rowmotion on 321-avoiding Permutations Ben Adenbaum [email protected] Dartmouth College (This talk is based on joint work with Sergi Elizalde.) We give a natural definition of rowmotion for 321-avoiding permutations, by translating, through bijections involving Dyck paths and the Lalanne–Kreweras involution, the anal- n 1 ogous notion for antichains of the positive root poset of the A − root system. We prove that some permutation statistics, such as the number of fixed points, are homomesic under rowmotion, meaning that they have a constant average over its orbits. Our proofs use a combination of tools from Dynamical Algebraic Combinatorics and structural properties of pattern-avoiding permutations. 1 Definitions 1.1 Permutations and lattice paths Let (321) denote the set of 321-avoding permutations of length n. We can represent Sn π n(321) as an n n array with crosses in squares (i, π(i)) for 1 i n. We say that∈ S (i, π(i)) is an excedance× (respectively weak excendance, deficiency,≤ weak≤ deficiency) if π(i) > i (respectively π(i) i, π(i) < i, π(i) i). ≥ ≤ p It will be convenient to consider two different ways to draw Dyck paths. Let n (resp. y D n) be the set of paths from (0, 0) to (n, n) with steps N = (1, 0) and E = (0, 1) that stayD weakly above (resp. weakly below) the diagonal y = x. There are several known bijections between 321-avoiding permutations and Dyck paths. For π (321), we define E (π) p to be the path whose peaks occur at the weak ∈ Sn p ∈ Dn excedances of π, E (π) p to be the path whose valleys occur at the excedances of π, v ∈ Dn and D (π) y to be the path whose valleys occur at the weak deficiencies of π. These v ∈ Dn are all described in more detail in [2]. See the left of Figure 2 for an example of Ep(π) and Dv(π) for π = 241358967. The bijection that maps Ep(π) to Dv(π) is known as the Lalanne–Kreweras involution on Dyck paths [5, 6], which we denote by LK. 4 1.2 Rowmotion and rowvacuation Suppose that P is a finite poset. Define (P ) to be the set of antichains of P . Antichain rowmotion is the map ρ : (P ) A(P ) defined as follows. For an antichain A, let A ρ (A) be the minimal elementsA of the→ Acomplement of the order ideal generated by A. A There is an equivalent way of defining rowmotion as a composition of antichain toggles, as first studied in [1] and later expanded upon in [8, 9]. Specifically, for p P , the antichain toggle associated to p is the map τ : (P ) (P ) defined by ∈ p A → A A p if p A, \{ } ∈ τp(A) = A p if p / A and A p (P ), ∪ { } ∈ ∪ { } ∈ A A otherwise. In other words, τp removes or adds the element p, with the caveat that it can only be added if the resulting set is an antichain. Then, for any linear extension (p1, . , pk) of P , one can show that ρ = τpk τpk 1 . τp1 . A − Next we define rowvacuation, which was first studied in [3]. When P is graded of rank r, let τ = τ where P is the set of elements of P of rank i. This product is well defined, i p Pi p i as two antichain∈ toggles commute if and only if the associated elements are incomparable Q [9, Lemma 3.12]. The rowvacuation map is then defined as τr(τrτr 1) ... (τrτr 1 . τ1τ0). − − When studying rowmotion, it is common to look for statistics that exhibit a property called homomesy [7]. Given a set S and a bijection τ : S S so that each orbit of the action of τ on S has finite order, we say that a statistic on→ S is homomesic under this action if its average on each orbit is constant.